The present invention relates to a process for the production of aromatic hydrocarbons from an aliphatic hydrocarbon charge having 2 to 12 carbon atoms in the presence of a zeolitic crystalline catalyst composition. It more particularly relates to the synthesis of a mixture mainly consisting of benzene, toluene and xylenes, which can improve the octane rating of petroleum products. It also relates to a hydrocarbon conversion apparatus, in particular for performing the aforementioned process.
The valorization of low boiling point aliphatic fractions, such as LPG's, justifies the interest attached to processes for the conversion of said hydrocarbons, which are efficient, selective and economic, while also contributing to the formation of hydrogen as a byproduct.
The aromatic hydrocarbon production reaction has in particular been described in U.S. Pat. Nos. 3,760,024, 3,756,942 and 3,855,980 in the presence of a silica and alumina-based crystalline zeolitic catalyst of the MFI type, such as ZSM5 or ZSM12, optionally with a metal such as gallium in the skeleton or in the presence of a zeolitic catalyst containing a metal outside the skeleton and as described in French Patent 2374 283 and U.S. Pat. No. 4,175,057.
The elementary processes involved in the transformation of aliphatic hydrocarbons into aromatic hydrocarbons are mainly the dehydrogenation of paraffins, the oligomerization of unsaturated hydrocarbons and the cyclization of oligomers. The reaction is highly endothermic, the reaction rate being sensitive to temperature variations, and said successive reactions are accompanied by a deposition of coke on the catalyst and a reduction of the metallic oxides contained in said catalyst, which very rapidly deactivates the catalyst.
It is known from U.S. Pat. No. 4,224,298 to use a fluidized particle bed for heating the tubes in which the reforming of hydrocarbons takes place with a view to obtaining synthesis gas at a temperature of 750.degree. to 800.degree. C. In this case, the external temperature of the skin of the reaction tubes is approximately 975.degree. C. The fluidized bed is heated by the combustion of a fuel and a combustion supporter directly in the particle bed. This apparatus suffers from the disadvantage of only being able to operate at high temperatures, i.e. it lacks the flexibility to enable it to be used in all reforming procedures. Moreover, it suffers from all the disadvantages resulting from preforming combustion in a fluidized bed.
There can firstly be problems of the dispersion of the fuel within the bed, particularly when the diameter of the latter exceeds e.g. 3 meters and when it is occupied by a large number of tubes. Under these conditions, the distribution of heat over the tubes is no longer homogeneous and there are risks of post-combustion beyond the fluidized bed.
It can also be difficult to check and control the bed operating at above 1000.degree. C., particularly during transient phases. In this case, there is a risk of the solidifying of the complete bed linked with local overheating in poorly fluidized areas. Thus, for example, in the case of an emergency stoppage of the apparatus, if the fuel supporter flow has not been completely stopped, the oxidation of the fuel present in the bed can continue in a fixed bed. This leads to a significant bed temperature rise through the absence of any heat exchange with the tubes, which may even lead to the melting of the bed.
In addition, French Patent 2531 944 describes a reforming process using the vapor of a hydrocarbon charge, so as to obtain a hydrogen-containing gas as the essential constituent. The reaction zone is heated via a fluidized particle bed, which is in turn heated by burners mounted directly on the reactor wall, so that combustion takes place within the fluidized bed to the extent that the temperature thereof is at least 750.degree. C.
In addition, French Patent 2374 283 proposes a gallium zeolite composition used in a process for the production of aromatic hydrocarbons from an aliphatic hydrocarbon charge.